A83.9 Mosquito-borne viral encephalitis, unspecified

The ICD-10-CM code for Mosquito-borne viral encephalitis, unspecified is A83.9 (FY2026). It is a billable, claim-ready diagnosis code.

Classification

Chapter

1: Certain infectious and parasitic diseases (A00-B99)

Section

A80-A89: Viral and prion infections of the central nervous system (A80-A89)

Category A83

10 codes (9 billable)

FY2026 Status

Stable since FY2024

Also Known As

ICD-10-CM Alphabetic Index entries that lead to A83.9:

• Borna disease
• Encephalomyelitis › equine
• Disease, diseased › Borna
• Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute) › equine (acute) (infectious) (viral)
• Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute) › viral, virus › arthropod-borne NEC › mosquito-borne

U.S. Hospital Utilization

A83.9 appeared in too few sampled U.S. inpatient stays in 2023 to report a national estimate (AHRQ suppresses counts of 10 or fewer) — a rare inpatient diagnosis.

Source: National Inpatient Sample (NIS), Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality, 2016–2023. National survey-weighted estimates.

Official Coding Guidelines

HIV Infections (TOC entry)

a. Human Immunodeficiency Virus (HIV) Infections .............................................................. 19

Infectious agents causing diseases in other chapters (TOC)

b. Infectious agents as the cause of diseases classified to other chapters ................................. 22

Source: CMS — ICD-10-CM Official Guidelines for Coding and Reporting, FY2026

References

• CMS — ICD-10-CM Official Code Set and Guidelines, FY2026
• CDC/NCHS — ICD-10-CM Files and Browser Tool
• AHRQ — HCUP National Inpatient Sample, Frequencies by Diagnosis Code (2016–2023)